In a conventional general synchronous motor, a stator is formed by integrally projecting plural teeth from a ring-shaped yoke to its inner circumferential side. This stator is fabricated by laminating stator plates having plural teeth projecting to the inner circumferential side. It also comprises a stator core forming slots among these teeth, and windings are wound in these slots by distributed winding. The distributed winding is a winding method for winding distant teeth through slots. The rotor is composed by burying plural permanent magnets for magnetic poles in the outer circumference of the rotor core, and mounting a rotary shaft in the center.
In this way, by burying permanent magnets inside the rotor, the buried permanent magnet motor can utilize not only the magnet torque but also the reluctance torque, in which the reluctance torque is generated in addition to the magnet torque by the permanent magnets, as an inductance difference occurs between the inductance Ld in the direction of the d-axis which is a direction for coupling the center of the permanent magnet and the rotor center, and the inductance Lq in the direction of the q-axis which is a direction rotated 90 degrees of electrical angle from the d-axis. This relation is shown in formula (1).T=Pn{ψa X Iq+½(Ld−Lq)×Id×Iq}  (1)                 where Pn: number of pole pairs        ψa: interlinkage magnetic flux            Ld: d-axis inductance            Lq: q-axis inductance        Iq: q-axis current        Id: d-axis current        
Formula (1) shows a voltage equation of dp conversion. For example, in a surface magnet motor, since the permeability of the permanent magnet is nearly equal to that of air, both inductance Ld and Lq in formula (1) are nearly equal values, and the reluctance torque portion expressed in the second term enclosed in braces in formula (1) does not occur.
In addition to the magnet torque, by utilizing the reluctance torque, if desired to increase the torque of the driving motor, according to formula (1), it is enough to increase the difference of (Ld−Lq). The inductance L, which expresses the degree of ease of passing of magnetic flux, is proportional to N,Q (number of turns of teeth), and hence by increasing the number of turns on the teeth, the difference of (Ld−Lq) becomes larger, so that the reluctance torque can be increased. However, if the number of turns is increased in order to utilize the reluctance torque more, as the number of turns increases, the winding group projecting to the stator end surface, that is, the coil end becomes larger. Hence, to rotate and drive the motor efficiently, if attempted to make use of the reluctance torque, the coil end becomes larger, and the motor itself is increased in size.
In the distributed winding, moreover, by turning windings plural times, a winding ring is formed, and this winding ring is inserted into the teeth, and the periphery of the winding ring becomes longer than the periphery of teeth. Still more, in the distributed winding, since the teeth are wound through slots, the windings cross each other. Thus, in the distributed winding, the winding projects from the stator end, and the windings cross each other to increase the size of the coil end.
Hence, if attempted to drive the motor efficiently by making use of the reluctance torque, the motor size becomes larger. To the contrary, if the motor is reduced in size, the output of the motor drops.
In the air-conditioner, refrigerator, electric vehicle, etc., however, a motor of large output and small size is required.
Incidentally, the magnetic pole portion at the end of the teeth in the stator is formed wider in the peripheral direction.
Between the adjacent magnetic pole portions, however, since openings are formed for laying down windings in the slots, the interval of ends of teeth must be formed wider in the peripheral direction. That is, because of the distributed winding, an opening for inserting the winding ring in the teeth is needed.
Incidentally, the gap between the stator inner circumference and the rotor outer circumference is generally set uniform on the whole periphery except for the openings of the slots.
In such conventional constitution, at the stator side, since there is an opening for a slot between magnetic pole portions, an insulating portion in the peripheral direction is formed in the distribution of the magnetic flux leaving the magnetic pole portions, which produced a problem of occurrence of cogging torque during rotor rotation. At the rotor side, when the distribution of the magnetic flux leaving its outer circumference is brought closer to sine waveform, the cogging torque can be decreased, but since the gap between the stator inner circumference and rotor outer circumference is uniform, the magnetic resistance in this gap is constant on the whole periphery, and in the joining portions of the ends of the permanent magnets, the magnetic flux distribution changes suddenly, and the cogging torque increases. Thus, the cogging torque increasing factors are combined at the stator side and rotor side, and a large cogging torque is caused.